1. Field of the Invention
The invention relates to hydrophobicized copolymers in the form of aqueous dispersions or water-redispersible powders, to a process for their preparation, and to their use.
2. Background Art
It is known from the prior art that the addition of hydrolyzable vinylsilanes during the preparation of polymer dispersions can achieve improvements in properties when such polymer dispersions are used as binders for paints. For example, DE-C 2148457 (GB-A 1407827) describes the use of aqueous dispersions which comprise polymers having copolymerized silanol groups to improve wet-adhesion of coating compositions used in construction work to a very wide variety of substrates. EP-A 327376 describes vinyl ester-based polymer dispersions which contain a small proportion of ethylenically unsaturated alkoxysilane monomers. The polymer dispersions are recommended for the production of paints with good wet-abrasion. EP-A 327006 (U.S. Pat. No. 5,576,384) discloses low-VOC (“volatile organic compounds”) emulsion paints and renders which include copolymers having hydrolyzable silane units as binders.
WO-A 95/20626 discloses the modification of water-redispersible polymer powders by addition of non-copolymerizable organosilicon compounds.
A disadvantage of these silane-modified polymer dispersions is that when used in the form of paint they do not bring about improvement in wet adhesion generally, but instead produce quite dissimilar effects in different paint formulations. EP-A 1153979 improves upon this variability by modifying the polymers with a combination of copolymerizable silanes and ethylenically unsaturated epoxides.
EP-A 0352339 describes protective paints for concrete structures comprising copolymers of divinylpolydimethylsiloxane with acrylic or methacrylic esters and with vinyl- and acrylic-functional alkoxysilanes, in the form of a solution in organic solvents. EP-A 159894 relates to hot-curing acrylic resin solutions in organic solvents which are used for metal coating, in particular for stainless steel. To improve metal adhesion, in particular to inhibit flake-off of the coating during deformation of the coated metal parts, use is made of acrylic resin solutions of copolymers of (meth)acrylates, hydroxy- or epoxy-functional (meth)acrylates, vinyl-functional polysiloxanes, and vinyl-functional silanes. To improve adhesion, the polymers are post-crosslinked and heat-cured, using organic tin compounds as crosslinking catalysts. A disadvantage of the solvent-containing and hot-curing coating compositions is that the solvent content and thermal crosslinking prevents their use as binders for polymer renders or in internal or external paints for the painting of buildings.
EP-B 771826 describes aqueous binders for coatings and adhesives based on emulsion polymers of vinyl esters, acrylic or methacrylic esters, or vinylaromatics, which comprise, as crosslinkers, polysiloxanes having unsaturated radicals such as vinyl, or acryloxy or methacryloxy groups. The organic monomer is emulsion polymerized, and the silicone is added at a particular juncture during the reaction. Additives which may be added subsequently include low-molecular-weight, polymerizable or non-polymerizable silanes, which permit further subsequent crosslinking of the polymer with organotin compounds. EP-A 943634 describes aqueous latices for use as coating compositions, which are prepared by copolymerizing ethylenically unsaturated monomers in the presence of a silicone resin containing silanol groups. Interpenetrating networks (IPNs) form from the polymer chains and polysiloxane chains. EP-A 1095953 describes silicone-grafted vinyl copolymers where a carbosiloxane dendrimer has been grafted onto the vinyl polymer.
Although the last-mentioned emulsion polymers are suitable for outdoor applications, their wet-abrasion resistance remains unsatisfactory. Another disadvantage of the silicone-modified emulsion polymers described in the prior art is a marked susceptibility toward hydrolysis and toward uncontrolled crosslinking. In many applications, crosslinking is desirable and is indeed reinforced by subsequent addition of silane and catalyst, but in paint dispersions or in coating-compositions, such crosslinking is the cause of undesirable gelling, “specks” and insolubles content. In addition, the silicone-containing emulsion polymers known hitherto are often not alkali-resistant, since silicones are known to be unstable under alkaline conditions. As a result, in the systems previously described, hydrophobic properties and other useful properties associated therewith fall away very sharply after a period of time. Finally, the introduction of large amounts of silanes or silicones results in unsatisfactory particle size distribution in the emulsion polymers, i.e. the particles become too large and the polymer becomes inhomogeneous, which may result in supernatant formation or phase separation.